Antinode weight assembly for rotor blades



April 1970 J. SCHMIDT 3,508,843

ANTINODE WEIGHT ASSEMBLY FOR ROTOR BLADES Filed Aug. 1. 1968 2 Sheets-Sheet 1 2e 33 23 25 FIG. 5

ZNVENTOR. JACOB SCHMIDT BY Agent WWW/ Attorney A il 28, 1970 J. SCHMlD T 3,508 8 ANTINODE WEIGHT ASSEMBLY FOR ROTOR BLADES Filed Aug. 1, 1968 2 Sheets-Sheet 2 FIG. 2

INVENTOR. JACOB SCHMIDT BY WMWI/ Attorn y United States Patent O 3,508,843 AN TIN ODE WEIGHT ASSEMBLY FOR ROTOR BLADES Jacob Schmidt, Van Nuys, Calif., assignor to Lockheed Aircraft Corporation, Burbank, Calif. Filed Aug. 1, 1968, Ser. No. 749,535 Int. Cl. B64c 11/8, 27/46 US. Cl. 416-144 7 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention The present invention relates to rotor blades and more particularly to means for dampening the harmonic modes of a rotor blade.

In tuning the blades for aircraft rotors, and particularly for articulated rotors, it has been found that the nodal characteristics of a blade can be changed with the utilization of an adjustable antinode weight located within the blade. This is accomplished by positioning the weight at a particular location on the blade that does not coincide with the even multiples of the blade rotating frequency. This antinodal positioning dampens the natural frequency of the blade, thereby reducing the vibrations in the blade and the airframe structure.

Description of the prior art Heretofore, prior weight means have been utilized for balancing or tuning rotor blades. However, such weight means suifer from various limitations and disadvantages, which have precluded their widespread acceptance in practical use.

One such means is exemplified by the US. Patent No. 2,430,948. The rotor blade is disclosed in that patent is provided with a plurality of holes or sockets located at its leading edge to receive a plurality of small balancing weights. However, since the leading edge is the main load carrying portion of the blade, the requirement for providing holes in its lower surface introduces severe stress concentrations, making the blade unsuitable for use in present day vehicles.

Another weight balancing means of the prior art is shown in US. Patent No. 2,613,893. In that disclosure an aileron is provided with cavities for receiving a low melting material such as lead. However, the open ended nature of these cavities makes them undesirable for use in todays high rotational speed rotors, since the lead tends to creep through the open ends under loads encountered. Furthermore, such a teaching could not be adapted to the leading edge of the main rotor blade, there being no means shown to introduce the lead into or remove it from the cavities.

SUMMARY OF THE INVENTION This invention overcomes the deficiencies of the prior art by providing a rotor blade tuning apparatus for adding or removing Weight within the contour of a stressed skin rotor blade without affecting the continuity of the main load carrying members of the blade. The apparatus "ice includes a fitting, which is usually machined but which could readily be cast or otherwise fabricated, installed beneath the skin portion of the rotor blade near the leading edge, the fitting having cavities defined therein, a filler tube communicating with the cavities and extending through the skin portion of the blade near the blade trailing edge, and an electric heating element extending over the fitting to heat the wall of the cavities whereby a low temperature solidifying alloy may be added to or removed from the cavities through the filler tube.

Thus, the apparatus fulfills a primary purpose of the invention, i.e., providing means for tuning a rotor blade which substantially precludes the introduction of stress concentrations to the skin portion of the leading edge of the blade.

Another object of the invention is to provide an improved mass balance rotor blade construction which provides secure anchorage of the balancing mass in the blade and secures maximum utilization of the mass from the standpoints of both structural strength and balancing affeet.

A further object of the invention is to provide a rotor blade tuning apparatus with a heating element to permit ease of adding or removing a balancing mass from the apparatus without destroying the continuity of the blade contour and structural load path.

Various other objects and advantages will appear from the following description of one embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a bottom perspective view of a blade section (in phantom) having an antinode weight apparatus installed therein;

FIGURE 2 is a fragmentary top plan view of the antinode weight apparatus with certain parts broken away to show the internal structure of the apparatus; and

FIGURE 3 is a cross sectional View of the antinode weight apparatus, taken along lines 3-3 of FIGURE 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the embodiment of FIGURE 1, the antinode weight assembly, shown generally at 10, includes a machined fitting 11 located within the stressed skin interior of a rotor blade 13 adjacent the leading edge 15. The advantage of positioning the fitting 11 near the leading edge is that less weight is needed at the leading edge for balancing purposes. Fitting 11 is also positioned near the outboard end 17 of the blade 13 for the maximum inertial effect for the weight of the fitting 11, although it can be positioned at any length which provides a maximum dampening effect. Of course, a plurality of antinode weights could be utilized in various positions in a single blade, if a plurality of nodes were present.

A filler tube 19 is attached to the fitting 11 and extends rearwardly to the skin portion of the bottom surface of the blade 13, near the trailing edge 21, where it forms a port 22. The filler tube 19 thus provides an external communication with the interior of the fitting 11. It is also noted that by being located entirely within the blade 13, except the structure defining the port 22 located near the trailing edge 21, the assembly 10 introduces no stress concentrations to the surface of the main load carrying members of the blade located near the leading edge 15.

The fitting 11 also includes a plurality of pockets 23 formed on the lateral side portions 24 on both sides of the fitting 11. These side portions 24 are provided to add structural strength and stability to the assembly. The pockets 23 are provided for weight saving purposes while maintaining structural integrity. If desired, the pockets 23 may have honeycomb or the like inserted therein for supporting the blade skin by an appropriate bonding technique. Also located on each side of the fitting 11, between the two rows of pockets 23, is a heating element or blanket 25. Each heating blanket 25, being bonded to the central portion 26 of the fitting 11, includes a plurality of heating coils, not shown, located therein which operates electrically in a conventional manner. An electric wire 27 is clipped to each heating blanket at 29, and extends along the exterior of the filler tube 19 until it approaches the port 22. At that point, the wire 27 extends through a hole 31 pierced in the tube 19, and into the interior of the tube 19 for connection to an outside electrical source (not shown).

As shown in FIGURES 2 and 3, the heating blanket 25 extends over the fitting central portion 26 and also over the inner side walls 33 of the pockets 23. With this structure, the entire central core 26 of the fitting 11 is substantially surrounding on both sides by the heating blankets 25.

A pair of parallel elongated cavities 35 are defined within the central core 26 and extend substantially the entire length of the fitting 11. The cavities 35 are machined through one end 37 of the fitting 11, which, in turn, is sealed by caps 39. The filler tube 19 communicates with the cavities 35 through a port 41, while a pair of channels 43, located at opposite ends of the cavities 35, provide communication between the two cavities 25.

The purpose of the cavities 25 is to contain a mass of low temperature, high density material. Various low melting point metal may be used for the mass, such as lead, which will be discussed as being representative. The introduction of the lead into the cavities 25 is accomplished by tilting the blade on its outboard end and holding the blade in the desired attitude for gravity filling of the cavities. If a portion or all of the mass of lead is desired to be removed from the blade, the blade is tilted on its inboard end. The heating blankets 25 are then energized to provide sufiicient heat to melt lead in the central core 26' and to permit it, through gravity, to flow out of the cavities 25 through the filler tube 19.

It is also noted that the closed ends of the cavities 35 are located on the outboard side of the fitting 11. Such a configuration prevents any creeping of the lead in the cavities because during rotation, the lead, through centrifugal force, will remain at the closed ends of the cavities.

Although the assembly has been described as functioning as an antinode weight assembly, it could also be adapted for mass balancing purposes.

What is claimed is:

1. A weight assembly for rotor blades comprising:

a fitting located within the interior of the rotor blade, said fitting having a central portion defining a cavity enclosed therein;

means for providing said cavity with communication to the exterior of said blade; and

electric heating means attached to said central portion of said fitting for heating the interior surfaces of said cavity.

2. The invention in accordance with claim 1 wherein said means includes a conduit connected to said fitting and communicating with said cavity, said conduit extending through the skin surface of the rotor blade.

3. The invention in accordance with claim 2 wherein said conduit extends through the skin surface of said rotor blade adjacent the trailing edge thereof.

4. The invention in accordance with claim 3 wherein said conduit is located inboard of said cavity.

5. The invention in accordance with claim 1 wherein said electric heating means is bonded to said central portion of the fitting.

6. The invention in accordance with claim 1 wherein an electrical conductor is attachced to said electric heating means and extends through a portion of said conduit for connection to an external power source.

7. The invention in accordance with claim 2 wherein the cavity is defined by a pair of elongated cylindrical chambers communicating With each other and with said conduit.

References Cited UNITED STATES PATENTS 1,894,276 1/1933 Lampton. 2,085,769 7/ 1937 Scott. 2,648,388 8/1953 Haines et al. 3,002,718 10/ 1961 Hackenberger. 3,323,597 6/ 1967 Longobardi et al.

FOREIGN PATENTS 192,528 2/ 1923 Great Britain.

EVERETTE A. POWELL, JR., Primary Examiner 

